Traditionally, a resin tube is connected with a metallic pipe which have a bulge or spool on its end made by a bulge-forming method or spool-forming method (these are methods to form a radially enlarged portion with nearly constant thickness on the pipe) by inserting the resin tube over the metallic pipe so as to be fitted together by pressure (For example, Patent Document 1).
In the above connected state where the resin tube is fitted by pressure onto the metallic pipe having a bulge or spool, the resin tube is enlarged in the diameter at a position where the radially enlarged portion is provided on the metallic pipe, and the diameter contraction action of the resin tube (restoration action of the resin tube trying to return to its original shape) develops a tightening strength to ensure a level of fluid tightness.
However, such a conventional connection structure for the resin tube has a drawback that the resin tube becomes declined in the tightening strength due to its thermal deterioration across the ages or the like and thereby the fluid tightness between the resin tube and the metallic pipe cannot be secured, hence a leakage of fluid may occur after a relatively shorter period of time.
Patent Document 1: Japanese Laid-open Patent Publication No. 9-126374 (See FIG. 5.)